1. Field of the Invention
This invention relates to an relates to an apparatus and method for controlled fatigue testing of a specimen wherein a magnitude of a load force applied to the specimen is varied over time so that the load force upon the test material is maintained constant during physical property changes of the test material.
2. Description of Prior Art
It is crucial to be able to create accurate and stable loading conditions during fatigue testing of a specimen or sample material. According to conventional apparatuses and methods, arbitrary force profiles cannot be generated without elaborate tuning of a force controller designed for a specific class of materials, which defeats the purpose of the testing activity since such conventional apparatuses and methods require prior knowledge of the physical properties of the specimen or material. With such conventional apparatuses and methods, even if they could be properly tuned, it would be very difficult to compensate for slow dynamic changes in the physical properties of the specimen, such as those caused by geometric deformations and propagation of cracks, which can lead to undetected instability of loading conditions.
It is important to distinguish between the control mechanisms of fatigue testing systems and those of other control systems, such as numerical controllers that are employed in machine tooling systems, temperature controllers, liquid level controllers and the like. In such other control systems, the control input variable, such as pulse frequency, coil current or valve position, is independent of the controlled variable, such as position, temperature and liquid level. However, in fatigue testing systems, the control variable, such as displacement, and the controlled variable, such as the load, interact through the unknown test specimen. The displacement variable plays a dual role, a control variable and a response variable of the test. Thus, the load becomes a test signal that directly depends upon the physical properties of the test specimen or other test material.
In some conventional fatigue testing equipment controlled variables other than displacement are used to control the load on the test specimen, which limits the force profiles that can be generated and often results in overheating of drive motors. Also, such conventional fatigue testing apparatuses use torque motors which provide relatively low positioning resolution, due to analog control circuits, and thus less stable load profiles.
U.S. Pat. No. 5,154,085 discloses a tension type dynamic viscoelasticity measuring apparatus which applies a prescribed force to a viscoelastic test material. Over time, due to molecular relaxation effects, the applied force diminishes. In order to correct for an anticipated relaxation-induced or time-dependent reduction in the force acting on the test specimen, the apparatus taught by the '085 patent exerts additional displacement or extension at a magnitude which is computed as a function of time. The control system taught by the '085 patent is considered to be open loop with respect to the tensile force. Since the apparatus taught by the '085 patent does not measure the actual force acting on the test specimen, adjustments in the specimen displacement tend to promote relaxation-induced force reductions. According to the apparatus of the '085 patent, the accuracy of the actual force magnitude relies on the predictive power of the time function.
It is well known that the intensity of stressed-relaxation of a polymer depends upon its molecular structure and its physical state. The number of common types of polymers and their corresponding physical states are so large that a preset time function cannot account for such variance.
The displacement-based feedback control taught by the '085 patent cannot be used to generate constant load magnitude which is essential for fatigue testing. The load prescribed by the moving mechanism taught by the '085 patent diminishes not only due to stress relaxation, a molecular process, but also due to cracks and other plastic deformation processes which cannot be compensated for by any known time function. The force acting on a polymer specimen under fatigue testing usually diminishes due to dimensional changes of the specimen, such as thinning, which cannot be compensated for through a time function. Fatigue testing involves complex specimen geometries, such as a plastic pipe section or a fused joint, wherein actual response to the applied force must be measured and corrected in real time. Fatigue testing requires intermittent application of tensile force and/or compressive force, thus the motion mechanism and control taught by the '085 patent is ineffective.